1. Field of the Invention
The present invention relates to electronics, and, in particular, to integrated circuitry.
2. Description of the Related Art
Integrated circuits are used for a wide variety of applications. For example, part of an integrated circuit (IC) can be designed to function as a decoder of information encoded into a received analog signal. What we commonly think of as digital signals are, in reality, analog signals that represent the digital information. In order to decode the information, the processing of the received signal may include one or more thresholding steps in which the magnitude of a signal is compared with a DC reference voltage signal that should ideally be noise-free. The analog reference signal is typically acquired from off-chip and is bused throughout the chip as needed for the decode processing. As the analog reference signal is bused throughout the chip, it may be exposed to electromagnetic coupling with other, possibly noisy signals that are also present on the chip. This electromagnetic coupling can induce noise in the analog reference signal, which may adversely affect the accuracy of the thresholding steps, thereby leading to errors in the decode processing.
One way to reduce such noise in the analog reference signal is to design the layout of the IC such that the conductor for the analog reference signal is sandwiched between layers containing existing structure designed to carry the power Vdd (i.e., the power rail) and the power ground Vss (i.e., the ground rail) for the IC. This solution effectively shields the analog reference signal from noise from signals on the opposite sides of the power and ground rails, but the shielding structure itself is not sufficiently quiet. Although, ideally, the power signal Vdd and the ground signal Vss should be perfectly constant, noise-free signals that are provided by quiet external sources, in reality, there is a tendency of the potentials on the power and ground rails to change as other components on the IC draw current from those rails, due to the finite impedance of those rails, their packaging parasitics, and their corresponding external sources. This phenomenon, which is referred to as ground bounce, can lead to an unacceptably high level of noise in the analog reference signal as a result of electromagnetic coupling between the signal conductor and the noisy power and/or ground rails.